Ralph T. Wiedmann

Rotationally resolved threshold photoelectron spectrum of the methyl radical

We report the rotationally resolved, one‐photon threshold photoelectron spectrum of the methyl radical, CH3, produced by supersonic‐jet, flash pyrolysis. Only rotational transitions with ΔK=0, ±2 are observed and this result is shown to be consistent with photoionization selection rules in D3h symmetry. Assignment of the threshold photoelectron spectrum results in an adiabatic ionization potential of 79 349±3 cm−1.

Rotationally resolved photoionization of H2O

A rotationally resolved one‐photon threshold photoionization spectrum of jet‐cooled water (H2O and D2O) has been obtained by pulsed field ionization of extremely high‐n Rydberg states. Observed spectral intensities for both vibrationless (0,0,0) and vibrationally excited (1,0,0) water cation show a strong propensity for ΔN=0, ±1 transitions. In contrast to earlier work on O2 and HCl, the lack of large ΔN transitions suggests that ionization occurs with only small angular momentum transfers between the core and photoelectron. The presence of both type A and type C ‘‘symmetric top’’ transitions varies from the conclusions of a recent MQDT analysis of H2O photoionization, which predicts only type C transitions. Rotational analysis of the spectra yields improved ionization potentials for both H2O and D2O. The ionization potential of the (1,0,0) vibrational level provides a direct measurement of the symmetric stretch fundamental in H2O+ which is in excellent agreement with an earlier indirect determination. Ro...

Rotationally resolved threshold photoelectron spectra of OH and OD

The results of combined experimental and theoretical studies of the rotationally resolved photoelectron spectra of OH and OD following single‐photon ionization are presented. The measured zero‐kinetic‐energy (ZEKE) spectra were obtained using pulsed field ionization in conjunction with a vacuum ultraviolet laser source. The OH^+ and OD^+ (X ^3Σ^−, v^+=0) rotational distributions were studied over the range 95.0–95.4 nm. Agreement between the observed and calculated spectra is very encouraging. Improved values for the ionization potentials of OH and OD (104 989 and 105 085 ± 2 cm^(−1), respectively) are reported and the unusual dynamics favoring ΔN<0 transitions are discussed.

Anomalous branch intensities in the threshold photoionization of HCl

The rotationally resolved threshold photoionization spectrum of HCl has been observed for ionization into both spin‐orbit components of the ground electronic state of HCl+. The data indicate an extreme asymmetry in the apportioning of angular momentum between the escaping photoelectron and the ion core. Observed transitions in which the core rotation decreases are found to be heavily favored compared to those where the core rotation increases by an equal amount. A mechanism of increased negative branch intensity due to field‐ or dipole‐induced mixing of Rydberg series converging to higher ion rotational levels is proposed as a possible explanation. Direct observation of the transition HCl (X 1Σ+,J‘=0)→HCl+ (X 2Π3/2,J+= (3)/(2) ) yields an ionization potential of 102 802.8±2 cm−1 for HCl.

High-resolution threshold photoionization of N2O

Pulsed field ionization (PFI) has been used in conjunction with a coherent vuv source to obtain high‐resolution threshold photoelectron spectra for the (000), (010), (020), and (100) vibrational states of the N2O+ cation. Simulations for the rotational profiles of each vibronic level were obtained by fitting the Buckingham–Orr–Sichel equations [A. D. Buckingham, B. J. Orr, and J. M. Sichel, Philos. Trans. R. Soc. London, Ser. A 268, 147 (1970)] using accurate spectroscopic constants for the ground states of the neutral and the ion. The relative branch intensities are interpreted in terms of the partial waves of the outgoing photoelectron to which the ionic core is coupled and in terms of the angular momentum transferred to the core. The PFI technique also allows us to report an improved value for the ionization potential of N2O of 103 963±5 cm−1.

Ion rotational distributions for near‐threshold photoionization of H2O

Ion rotational distributions for single‐photon VUV photoionization of the 1b_1 orbital of the X ^1A_1 ground state of the jet‐cooled water are reported. These spectra reveal significant type a transitions which are seen to arise from odd angular momentum components of the photoelectron matrix element. The resulting photoionization dynamics are quite nonatomic‐like.

Direct determination of the adiabatic ionization potential of NO2 by multiresonant optical absorption

Abstract The adiabatic ionization threshold of NO 2 is observed by direct optical excitation in a three-color, triply resonant, three-photon transition from vibrationless levels of the bent ground state through the vibrational ground state of the linear Rydberg state to the ground state of the ion. The overall energy for this process for rotationless molecules is estimated to be 77320 ± 20 cm −1 . The bent-to-linear two-photon transition to the Rydberg state is facilitated by a first-photon resonance with levels of NO 2 s mixed low-lying excited states. At higher energies, following intermediate selection of vibrationally excited 3pσ Rydberg states, transitions to series of high Rydberg states are observed uniformly converging to respective vertical ionization potentials. These transitions are marked by sharp bands below and in some cases above the adiabatic ionization threshold. Intensities suggest efficient autoionization.

Rotationally resolved photoionization of polyatomic hydrides: CH3, H2O, H2S, H2CO

Combined theoretical and experimental studies of rotationally resolved photoelectron spectra for single‐photon ionization of the outermost valence orbitals of H2O, H2S, H2CO, and CH3 are reported. Agreement between calculated and measured spectra is very encouraging. Both show that photoionization dynamics is very molecular in origin for H2O, H2S, and H2CO but quite atomiclike for CH3. Parity selection rules and the angular momentum composition of the photoelectron are used to illustrate the dynamical aspects of photoionization of polyatomics as molecular symmetry changes in a group of structurally related systems.

Vibronic coupling in the X̃ 2Π and à 2Σ+ states of HCN+

The vibronic structure associated with the X 2Π and A 2Σ+ states of HCN+ have been examined by high resolution threshold photoelectron spectroscopy using coherent vacuum ultraviolet radiation and pulsed field ionization techniques. Rotationally resolved spectra for 12 vibronic bands are presented from which vibronic symmetry assignments and cation spectroscopic constants are derived. The latter are compared to the ab initio calculations of Koppel et al. [Chem. Phys. 37, 303 (1979)] who calculated the vibronic structure of HCN+ based on strong X–A interstate coupling. Finally, the observed rotational branch structure is used to infer the ionization dynamics of strongly coupled vibronic levels for which the Born–Oppenheimer approximation is not valid.

Single‐photon threshold photoionization of NO

Single‐photon threshold photoionization spectra for jet‐cooled NO have been measured for the v^+=0 and 1 vibrational levels of the X ^1Σ^+ ground state of NO^+. The NO^+ rotational state distribution for the v^+=0 level is shown to be perturbed by nearby autoionizing levels, whereas the v^+=1 level exhibits a cation rotational distribution which is in near quantitative agreement with calculated spectra near threshold. Only small changes in total angular momentum are observed (‖ΔJ‖=‖J^+ − J‘‖≤5/2) even though a wide range of photoelectron angular momenta (l=0–3) are predicted to contribute to the near‐threshold photoelectron continua. The present results are also discussed in light of recently published two‐photon threshold photoionization spectra of NO which exhibit nearly identical NO^+ rotational state distributions.